In systems and devices implementing near field communication (NFC), an NFC reader/writer device may communicate with several NFC enabled cards/tags. Such NFC cards/tags may be passive, meaning that they have no power. In a typical implementation, the NFC reader/writer device attempts to poll or determine which NFC cards/tags are within its environment.
An antenna of the NFC reader/writer device may emit a radio frequency (RF) field. When a passive NFC card/tag is brought into the RF field of the NFC reader/writer device, the passive NFC card/tag receives power and begins communications with the NFC reader/writer device.
Before such communication is established between the NFC reader/writer device and the passive NFC card/tag, the NFC reader/writer device detects the presence of the passive NFC card/tag. Typically, detecting the presence of the passive NFC card/tag may be performed by the NFC reader/writer device emitting the RF field continuously, or in order to save power, emitting the RF field for a short duration of time. A change in voltage level may be seen at an antenna and/or transmitter pins of the NFC reader/writer device. This change of voltage level may be indicative of the presence of a passive NFC card/tag.
Several issues exist with the typical detection of a NFC card/tag. Although, the RF field may be emitted in short durations, a power amplifier providing the necessary energy may have to emit maximum power during transmission of the RF field. The maximum power is used to power a passive NFC card/tag. This may result in higher power dissipation by the NFC reader/writer device.
Another issue may be false indications. When voltage levels change at transmitter pins or an antenna of the NFC reader/writer device, this may indicate the presence of an NFC card/tag. When a metallic or magnetic object that is a non NFC card/tag enters the emitted RF field of the NFC reader/writer device, the voltage levels at the transmitter pins or antenna of the NFC reader/writer device change, and the NFC reader/writer device may falsely recognize the object as a valid NFC card/tag.